Project Summary and Abstract Background: Dysregulation of nitric oxide (NO) signaling is linked to various diseases including neurodegeneration, hypertension and stroke, heart disease, erectile dysfunction, gastrointestinal distress, and many forms of cancer. NO signaling begins in vivo with its synthesis by nitric oxide synthases (NOS). However, much is unknown regarding the catalytic and regulatory mechanisms of NOS enzymes. NOS enzymes catalyze the oxidation of arginine to NO and citrulline using oxygen and NADPH as cosubstrates in a two-step reaction with N[G]-hydroxyarginine, NHA, as an intermediate. Both steps occur in the heme-containing oxidase domain of NOS enzymes, which is fed electrons from a flavin-containing reductase domain. However, rate limiting electron transfer masks observation of the activated oxygen intermediates responsible for arginine and NHA oxidation. Knowledge of these intermediates is crucial to understand the catalytic mechanism of NOS enzymes. In Aims 1 and 2, these intermediates will be directly observed using novel spectroscopic techniques. NOS activity is regulated by variety of post-translational modifications (PTMs). Elucidation of the interplay between NOS modifications to control NO synthesis is a fertile area for research. In addition, targeting of PTMs instead of enzyme active sites is an orthogonal mechanism to treat diseases associated with dysregulation of NOS activity. Of the known modifications, inhibition of endothelial NOS (eNOS) by acetylation is the least characterized. Aim 3 will elucidate how acetylation works in concert with other PTMs to control eNOS activity. Specific aims: 1) Directly observe oxygen intermediates responsible for substrate oxidation. Photochemical inducible NOS (iNOS) enzymes will be designed wherein metallolabels deliver electrons rapidly to the heme upon excitation with light. These photochemical iNOS enzymes will then be utilized in flow-flash spectroscopic investigations, wherein oxygen is bound to a ferrous heme, activated by photoinduced electron transfer, and probed with a variety of spectroscopic techniques. 2) Use non-natural substrate and heme analogs to probe oxygen activation. Non-natural substrate and heme analogs will be used to probe the structure-function and thermodynamic-kinetic relationships of oxygen activation in NOS enzymes using the techniques developed in Aim 1 and standard NOS assays. 3) Determine the mechanism of eNOS inhibition by acetylation in the context of other post-translational modifications. The precise sites of eNOS acetylation will be determined by mass spectrometry and then the mechanism of eNOS inhibition by acetylation will be determined. The effect of acetylation on other PTMs within eNOS will also be examined.